The present invention relates to induction heating, and magnetic annealing using induction heating.
Ferromagnetic thin films are important components of many microelectronic devices and structures. For example, magnetic random access memories (MRAMs) are magnetic storage devices that store information in the form of magnetization of a ferromagnetic thin film that comprises part of a magneto-resistive sensor. Information can been written into the ferromagnetic thin film and read out by detecting a magneto-resistance of the sensor. MRAMs have been refined to allow increasingly large amounts of data to be stored in a non-volatile manner and are typically fabricated on wafers using conventional semiconductor fabrication techniques.
The deposited ferromagnetic thin films in MRAMs and other devices and structures often require magnetic annealing in order to impart certain desired magnetic characteristics to such a layer. Specifically, magnetic annealing refers to a process by which a ferromagnetic material undergoes exposure to an external magnetic field at elevated temperatures in order to increase the size of magnetic domains to increase permeability and to impart a particular magnetic orientation to the magnetic dipoles in the ferromagnetic material. The crystalline structure of a deposited ferromagnetic thin film is responsive to increased temperature. In particular, raising the temperature increases the vibrational moments of atoms forming the crystalline structure, imparts a randomness to the motion of these atoms, and places these atoms in a state that provides minimal resistance to the influence of an external magnetic field. An external magnetic field causes the magnetic dipoles of these atoms to be oriented along the axis of the applied magnetic field. Prior art systems for performing magnetic annealing include a vacuum oven or furnace or the like for providing the elevated temperatures required.
Other modern microelectronic circuit process technologies incorporate device structures having high sensitivities to thermal treatment. The high sensitivities are due to the precise definition of device regions. These regions may include ultra-thin ion implanted source and drain regions in a submicron complementary metal-oxide-semiconductor field-effect transistor circuit (CMOS FET), among others, and exposure of such regions to high temperature result in the degradation of device performance. Thus, material and process related temperature limitations prohibit the integration or incorporation of a wide range of possible device structures in the fabrication process. Applications such as high temperature treatment of embedded high voltage, high current, or high power microelectronic devices often require complete thermal isolation of process modules. An example of such a system is an embedded processor that is responsible for driving motors. The system might include drivers for driving the motors, and a plurality of high density logic circuits for controlling the operation of the motor. Both the drivers and the logic circuits typically have different thermal capacities and, therefore, complicated processes are required to embed the drivers and the logic circuits in a single system.
The incorporation of devices into mainstream processing also presents thermal challenges for integration of the technologies. Device examples include micro-electromechanical systems (MEMS), micromachines and microsystems. Various thin film structures for the devices may also require thermal treatment to stabilize the mechanical properties for use. However, a microelectronic process typically requires lower temperatures throughout its processing. The relatively high thermal energy generated in the thermal treatment of the MEMS tends to impact the CMOS processing of the system. Similarly, embedding elements, like radio-frequency components into high density CMOS, require different thermal treatments and, thus, require complicated processes to embed them together.
Furthermore, many device packaging applications require protection or packaging, often prior to final encapsulation or before being singulated, in order for the devices to function. Pressure sensors, accelerometers, optoelectronic device assemblies, and some microelectronics technologies require packaging or assemblies that utilize temperatures above 300–400° C., which temperatures could damage microelectronic circuitry.